hit it all ya want!!!
the air bag sensors are all "decelleromters" (i know the spelling is off!!) usually located in the middle of all vehicles. they work on a rate of slowing down, not impact. you can hit a deer and with a large vehicle and usually not deploy the airbag because the animal is not heavy enogh to cause a rate of deceration that is quick enough to cause the airbags to deploy. striking the bumper, or any other part of the vehicle for that matter, will not result in airbag deployment. if it does, then the airbag system was defective in the first place!! feel free to hit the bumper all day with a sledge hammer, the airbag won't go off!!